The present invention relates to a dry toner for developing electrostatic latent images to visible images, for useing electrophotography, electrostatic recording methods and electrostatic printing methods.
Conventionally, as a developer for use in these fields, a so called two-component dry-type developer is well known, which comprises carrier particles and toner particles. In the two-component dry-type developer, particle size of the toner particles is much smaller than that of the carrier particles, and the toner particles are triboelectrically attracted to the carrier particles and held on the surface of the carrier particles. Electric attraction between the toner particles and the carrier particles is caused by friction between the toner paraticles and the carrier particles. When the toner particles, which are held on the carrier particles, are brought near or into contact with the latent electrostatic images, electric fields of the latent electrostatic images work on the toner paraticles to separate the toner particles from the carrier particles, to overcome the bonding attraction between the toner particles and the carrier particles. As a result, the toner particles are attracted towards the latent electrostatic images in an ordinary case, or repulsed by the latent electrostatic images in the case of a reversal development, and the latent electrostatic images are thus developed to the visible images.
In the case of the two-component dry-type developer, only the toner particles are consumed as the development is performed. Therefore, it is necessary to replenish the toner particles in a course of the repeated development by a developing apparatus. Further, the carrier particles are required to charge the toner particles triboelectrically in such a way that the toner particles have desired polarity and a sufficient amount of charges for a long period of time during use.
In the case of the conventional two-component dry-type developer, it is apt to occur that resins contained in and released from the toner particles in course of a mechanical mixing of the toner particles and the carrier particles in the developing apparatus, eventually cover surfaces of the carrier particles. Once surfaces of the carrier particles are covered with resins, which are referred to as a "spent phenomenon", such particles no longer function as active carrier particles, that is, the carrier particles which contribute to the development. As a result, charging characteristics of the carrier particles in the conventional two-component dry-type developer deteriorate with the time in use. In the end, it is necessary to replace an entire developer by a new developer.
In order to prevent the spent phenomenon, a method of coating the surface of carrier particles with any of several different resins has been proposed.
In an ordinary case, the latent electrostatic images with positive charges on a photoconductor as in the case of an amorphous selenium photoconductor are developed by negatively charged toner particles, and latent electrostatic images with negative charges on a photoconductor as in the case of an organic photoconductor are developed by a positive charged toner particles. In the case of reversal development, the electrostatic latent images with positive charges are developed by positively charged toner particles, and latent electrostatic images with negative charges are developed by negatively charged toner particles.
The resin overcoated on the surface of the carrier particles must be selected so as to prevent the spent phenomenon, and to toriboelectrically charge the carrier particles to desired polarity. For instance, in case of carrier particles coated with fluorocarbon polymers, of which surface enegy is 10.about.15 dyne/cm and extremly low compared with other resins, the spent phenomenon occurs only slightly. However, since fluorocarbon polymers are on extremely negative side in terms of triboelectric charging series, carrier particles coated with fluorocarbon polymers can be only used for the toner particles which are to be charged to positive polarity.
In case of carrier particles coated with silicone polymers, of which surface enegy is 23.about.28 dyne/cm, the spent phenomenon occurs only slightly as the carrier particles coated with fluorocarbon polymers. Furthermorer, silicone polymers are on more positive side than fluorocarbon polymers in terms of totriboelectric charging series. Therefore, as disclosed in U.S. Pat. No. 4,584,254, (issued Apr. 22. 1986) the carrier particles coated with the silicone polymers can be used to both positively chargeable toner particles and negatively chargeable toner particles, and a successful scale merit can be expected for a mass production of the carrier particles coated with the silicone polymers.
As a curing condition of silicone polymer of which curing temperature is from 20.degree. C. (room temperature) to 200.degree. C. and is considerably lower when compared to 350.degree. C. for the fluorocarbon polymers, production of carrier particles coated with silicone polymers is considered to be inexpensive.
Furthermore, it is indispensable to reduce particle size of carrier particles, whose conventional size is 50.about.250 .mu.m, in order to improve quality of the developed image. In order to prevent the spent phenomenon to the toner particles of the reduced size it is necessary to be coated with a thin resin film on the surfaces of the carrier particles, and the silicone polymers are most suitable for this purpose.
However, this useful carriers coated with the silicone polymers have a defect that an ability of charging a sufficient amount of charges to the toner particles is inferior to the carrier coated with the fuluorocarbon polymers. Then, in order to use the carriers coated with silicone polymers together with the positively chargeable toner particles as the two component dry-type developer, many properties respect to chargeability of toner particles must be improved, such as a positive chargeability, a narrow charge distribution range of toner particles, running durability for a long time of period, short triboelectric charging time to charge a sufficient amount of charges for the toner particles, and maintenance of a toner charge level under widely varing environmental conditions, especially varing rerative humiditiy (R.H) conditions.
To enable the toner particles to retain the charge, it is proposed to utilize the triboelectric chargeability of polymers used as a main component of the toner particles, but the so adapted is low in chargeability, and the toner images obtained are apt to be fogging and obscure.
Then, an another method, in which a charge control agent is used for triboelectrically charging toner particles to the desired polarity and to the sufficient amount of charges, has been proposed.
As the charge control agents for giving positive chargeability, Olson (U.S. Pat. No. 3,647,696 issued Mar. 7. 1972) describes organic acidnigroshine salt, and Jacknow et. al., (U.S. Pat. No. 3,577,354 issued Mar. 4. 1971), describes a solid metal salt of a higher fatty acid. However, adhesion of fixed images to a suitable receiving sheet by the toner particles modified with the such organic salts is weak, and the such organic salts are apt to change the chemical quality, when the toner particles are triboelectorically charged by the carrier particles, and as a result the chargeability of the toner particles are decreased. Thomas et. al. (U.S. Pat. No. 3,893,935 issued Jul. 8. 1975) describes quaternary anmonium salt having long chain aliphatic hydrocarbons for the positive charge control agents. Even if toner particles incorporated with the above mentioned control agents have the high chargeability, it changes with environmental conditions, esecially with high temperature and high humidity conditions.
To overcome these defects of such quaternay anmonium salt, modifications of chemical structurs of quaternary anmonium salt have been proposed, such as an azoniabicyclooctanate (Burness et. al. U.S. Pat. No. 4,079,014 issued Mar. 14. 1975), organic quaternary anmonium sulfonate and sulphate (Lu et. al., U.S. Pat. No. 4,338,390 issued Jul. 6. 1982), alkyl pyridinium salt (Lu et. al., U.S. Pat. No. 4,298,672 issued Nov. 3. 1981), quatenary anmonium inner molecular salt (Barbett et. al., U.S. Pat. No. 4,752,550 issued Jun. 21. 1988), improved organic quaternary anmonium sulfonate (Kawagishi et. al., Japanese Laid-Open Pat. App. No. 60-169857 issued Sep. 3. 1985, App. No. 62-3259 issued Jan. 9. 1987, and App. No. 62-71968 issued Apr. 2. 1987). Although there are so many discriptions about improvements of the chargeability for the environmental conditions such as high temperature and high humidity in the above mentioned literatures, almost of the modified toner particles comprising proposed quaternary anmonium salt are combined with the carrier particles coated with fluorocarbon polymers which are on extremely negative side in terms of totriboelectric charging series.
When above mentioned modified toner particles are combined with the carrier particles coated with the silicone polymers which are a little more hydrohobic than the fluorocarbon polymers, chargeability of the toner particles change with widely varing environmental conditions. For example, in case of conditions of high temperature, and high humidity an amount of charges are decreased, and some defects are arised such as deposition of toner particles on a back ground, deterioration of resolution of developed images, separation of toner particles from the carrier particles. On the other hand, in case of conditions at low temperature and low humidity, some another defects are arised such as decreasement in image density, empashyses of edge efects, where only edges of latent electrostatic images are developed and solid area can not be developed perfectly.
Such shortcomings tend to be intesified when carbonblack having low electric resistivity (10.sup.3 .about.10.sup.6 .OMEGA.cm) is used as a colorant in the toner particles.